Summary The debilitating central nervous system (CNS) disease multiple sclerosis (MS), is characterized by an influx of peripheral immune cells that coincides with axonal demyelination and damage, resulting in pain and impaired coordination. The role of B cells in MS pathogenesis has emerged as studies showed that B cell directed therapies substantially reduce the formation of new inflammatory demyelinating lesions. Further, the majority of lesions from patients with established MS harbor B cells as well as reactive microglia. While these findings corroborate a pathogenic role for B cells, it is unclear which B cell function contributes to demyelination within the inflamed CNS. In addition, systemic B cell elimination can severely immunocompromise patients, establishing the need to develop more targeted therapies. Emerging evidence suggests that B cells of MS patients are potent regulators of pro-inflammatory cytokines, and these abnormal B cells can regulate the CNS innate immune system when they enter the CNS. Studies in this application will selectively and locally ablate B cells in the CNS using a novel caspase 9-mediated apoptosis transgenic tool in the well-studied experimental autoimmune encephalomyelitis (EAE) animal model of MS. Preliminary data indicate that targeting CNS B cells after the onset of symptoms in EAE reduces microglial reactivity and is accompanied by decreased myelin damage and functional impairment. Inflammatory signaling pathways in microglia from CNS B cell depleted EAE animals will be evaluated using advanced phospho-flow cytometry techniques. Demyelination and remyelination in the brain and spinal cord will be further studied in this powerful, novel animal model to test if CNS B cell ablation prevents further myelin injury and promotes myelin repair. The role of MS B cells in neuroinflammation will be determined by stimulating human microglia with MS patient-derived B cell secreted factors and analyzing transcriptional and translational inflammatory activity of microglial cells. B cell induced microglial activation will be confirmed in human MS brain lesions. Together, the proposed studies will provide novel insights into the complex immunopathology of MS that involves the interaction of CNS B cells and microglia. Completion of these studies will lead to novel immunomodulatory therapeutic avenues that reduce the disease burden of multiple sclerosis without immunocompromising the patient. The proposed training plan is sponsored by Dr. Robert H. Miller at the George Washington University School of Medicine and Health Sciences. The overall goal is to build a strong foundation for the PI, Julie J Ahn, to prepare her for a successful career as an independent scientist in the fields of neuroimmunology and neuroinflammation. The fellowship training plan includes the following goals: 1) enhance scientific knowledge and technical skills that integrate neuroscience and immunology, 2) present research findings through oral presentations and manuscripts, 3) mentor and lead young scientists, and 4) conduct research responsibly.